Pollution control devices are employed for example in motor vehicles like passengers' cars or trucks or in industrial applications to control atmospheric pollution. Such devices include a pollution control element. Exemplar pollution control elements include catalytic converters and diesel particulate filters or traps. Catalytic converters typically contain a ceramic monolithic structure having walls that support the catalyst. The catalyst typically oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in the engine exhaust gases to control atmospheric pollution. Selective Catalytic Reduction (SCR) catalysts work by chemically reducing NOx (NO and NO2) to nitrogen (N2). The monolithic structure may also be made of metal. Diesel particulate filters or traps typically include wall flow filters that are often honeycombed monolithic structures made, for example, from porous ceramic materials. The filters typically remove soot and other exhaust particulate from the engine exhaust gases. Each of these devices have a housing (typically made out of stainless steel) that holds the pollution control element.
In automobile applications, monolithic pollution control elements are often described by their wall thickness and the number of openings or cells per square inch (cpsi). In the early 1970s, ceramic monolithic pollution control elements with a wall thickness of 12 mils (304 micrometers) and a cell density of 300 cpsi (47 cells/cm2) were common (“300/12 monoliths”).
As emission laws became more stringent, wall thickness have decreased as a way of increasing geometric surface area, decreasing heat capacity and decreasing pressure drop of the monolith. The standard has progressed to for example 900/2 monoliths. With their thin walls, monolithic structures are fragile and susceptible to vibration or shock damage and breakage. The damaging forces may come from rough handling or dropping during the assembly of the pollution control device, from engine vibration or from travel or rough roads. The monoliths are also subject to damage due to high thermal shock, such as from contact with road spray.
There is another future trend that should be mentioned, which is to use catalyst carriers showing reduced compressive strengths. For example, for diesel particulate filters the trend goes to high porosity filter substrates with a reduced isostatic strength (for example 0.8 to 1 bar). Rectangular extruded substrates used as catalyst carriers in SCR systems for non-road and stationary applications like rail, marine and industrial applications have a compressive strength at temperatures below 45° C. of about 1 bar.
The ceramic monoliths have a coefficient of thermal expansion generally about an order of magnitude less than the metal housing which contains them. For instance, the gap between the peripheral wall of the metal housing and the monolith may for example start at about 4 mm, and may increase by about 0.33 mm as the engine heats the catalytic converter monolithic element from 25° C. to a maximum operating temperature—in the automotive industry—of about 900° C. to about 530° C. Even though the metallic housing undergoes a smaller temperature change, the higher coefficient of thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the monolithic element. Such thermal cycling typically occurs hundreds or thousands of times during the life of the vehicle. Typical temperatures in for example selective catalytic reduction catalysts in marine applications are 250° C. to 550° C.
To avoid damage to the monoliths from road shock and vibrations, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monoliths and the metal housings (thereby bypassing for example the catalyst), mounting mats are disposed between the ceramic monoliths and the metal housings. The process of placing the monolith within the housing is also called canning and may include such steps as wrapping a sheet of mat material around the monolith, inserting the wrapped monolith into the housing, pressing the housing closed, and welding flanges along the lateral edges of the housing. Other processes insert the monolith together with the wrapped mounting mat into the already closed housing by using pressure.
Typically, the mounting mat materials include inorganic fibers, optionally intumescent materials, organic binders, fillers and/or other adjuvants. Known mat materials, used for mounting a monolith in a housing are described in, for example U.S. Pat. No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.), U.S. Pat. No. 4,385,135 (Langer et al.), U.S. Pat. No. 5,254,410 (Langer et al.), U.S. Pat. No. 5,242,871 (Hashimoto et al.), U.S. Pat. No. 3,001,571 (Hatch), U.S. Pat. No. 5,385,873 (MacNeil), and U.S. Pat. No. 5,207,989 (MacNeil), GB U.S. Pat. No. 1,522,646 (Wood) published Aug. 23, 1978, Japanese Kokai No.: J.P. Sho. 58-13683 published Jan. 26, 1983 (i.e., Pat Appln Publn No. J.P. Hei. 2-43786 and Appln No. J.P. Sho. 56-1 12413), and Japanese Kokai No.: J.P. Sho. 56-85012 published Jul. 10, 1981 (i.e., Pat. Appln No. Sho. 54-168541). WO 2007/143,437 A2 discloses a multilayer mounting mat comprising fibers that may be sized to provide lubricity and to protect the fiber strands during manufacturing of the mat. WO 94/16,134 discloses a flexible nonwoven mat comprising ceramic oxide fibers. To facilitate processing and separation of the fibers, an antistatic lubricant may be provided. WO 2009/048,859 discloses a method of making mounting mats. In one embodiment the fibers are impregnated.
Mounting mat materials should remain very resilient at a full range of operating temperatures over a prolonged period of use. At the same time they should be designed such that the pollution control system may easily be mounted which may be a problem, if for example fragile structures need to be mounted and/or if squared substrates, where only a limited pressure can be applied, need to be mounted. Particularly, when multiple substrates are mounted together in one housing the canning force needs to be higher. The ease of mounting strongly depends on the cold peak pressure or cold peak compression (P0) of the mat. In the following only cold peak pressure will be used for this parameter. Mounting mats with high cold peak pressure may be difficult to mount at the desired mat mount target density. Consequently, there is a need for mounting mats with reduced cold peak pressure which will still provide sufficient holding force in hot applications to prevent monolith movement.
In view of the above, there is a need for further improvements concerning the ability of easily mounting exhaust gas after-treatment element or pollution control systems.